Force rebalance accelerometers which include a proof mass suspended between one or more magnet assemblies are generally known in the art. Examples of such accelerometers are disclosed in U.S. Pat. Nos. 4,182,187; 4,250,757; 4,394,405; 4,399,700; 4,400,979; 4,441,366; 4,555,944; 4,555,945; 4,592,234; 4,620,442; 4,697,455; 4,726,228; 4,932,258; 4,944,184; 5,024,089; 5,085,079; 5,090,243; 5,097,172; 5,111,694; 5,182,949; 5,203,210; 5,212,984; 5,220,831; and Re. 34,631 all of which are hereby incorporated by reference. Such force rebalance accelerometers normally include a proof mass having a reed or flapper formed from amorphous quartz, suspended by one or more flexures between stators having permanent magnets to enable the proof mass to deflect in response to forces or accelerations along a sensitive axis, generally perpendicular to the plane of the proof mass. The proof mass also typically includes at least one torquer coil secured to the reed which functions as an electromagnet. At rest, the proof mass is normally suspended equidistantly between upper and lower excitation rings. Electrically conductive material forming pick-off capacitance plates, is disposed on opposing sides of the proof mass to form capacitive elements with the excitation rings. An acceleration or force applied along the sensitive axis causes the proof mass to deflect either upwardly or downwardly which causes the distance between the pick-off capacitance plates and the upper and lower excitation rings to vary. This change in the distance between the pick-off capacitance plates and the upper and lower excitation rings causes a change in the capacitance of the capacitive elements. The difference in the capacitances of the capacitive elements is thus representative of the displacement of the proof mass along the sensitive axis. This displacement signal is applied to a servo system that includes the torquer coils which function, in combination with a current applied to the torquer coils and the permanent magnets, to return the proof mass to its null or at-rest position. The magnitude of the drive currents applied to the torquer coils, in turn, is representative of the acceleration or force along the sensitive axis.
One problem encountered in this type of accelerometer results from the use of a gas, usually a mixture of neutral gases such as helium and nitrogen, utilized to fill the accelerometer to provide gas damping for the proof mass. Under extreme acceleration conditions, it has been found that the gas damping can result in an overshoot condition when the extreme acceleration condition is removed.
In addition, it has been discovered that in certain power off situations the reed, which has a highly polished surface will have a tendency to stick or latch up to the highly polished surface of the stator thereby increasing turn on times for the accelerometer. One approach to solving this problem is described in U.S. Pat. No. 4,825,335 wherein a rectangular moveable capacitor plate, which is suspended on each side by fingers, is provided with a number of air passages extending through the plate along with grooves in the plate leading up to the air passages to facilitate the flow of air. However etching holes in the flapper of the above described accelerometer is not a practical solution to this problem.